Intrusion detector



June 25, 1968 w, G. KAI-M JR.. ETAL. 3,390,388

INTRUS ION DETECTOR Filed March 24, 196 5 Sheets-Sheet l June 25, 1968 w. G. KAHL, JR.. ETAL 3,390,388

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United States Patent O 3,390,388 INTRUSION DETECTOR William G. Kahl, Jr., Brookfield, John Gally, Jr., Bethel, and Peter F. Cavanaugh, Westport, Conn., assignors to Arrowhead Enterprises, Inc., Bethel, Coun., a corporation of Connecticut Filed Mar. 24, 1965, Ser. No. 442,313 16 Claims. (Cl. 340-258) This invention relates to intrusion detectors or burglar alarms and, more particularly, to such detectors of the infrared photoelectric type.

Infrared photoelectric intrusion detectors are widely used to protect businesses and other institutions from intruders. These detectors customarily comprise a transmitter which is designed to project a beam of infrared light across the area to be protected to a receiver unit. At the receiver the light beam is focused onto the surface of a photosensitive detector. The detector is electrically connected in such a manner that a relay is kept in its energized position, with the relay contacts open, so long as the beam remains unbroken. The relay contacts, in turn, control an alarm of any suitable type which may be energized from an independent source. As the infrared beam is substantially invisible, it cannot be seen by an intruder. If an intruder passes through the beam, the light received by the detector is cut off, de-energizing the relay. The relay contacts close, activating the alarm.

In installing this type of intrusion detector, it is important that it be positioned to cover those areas most apt to be traversed by an intruder. These areas will, in all probability, be the same portions of the establishment which are high trafc areas when the system is not activated. These might include, for example, the aisle-ways of stores and the passageways through storage areas. It may also be desired to use mirrors in the system to enlarge the protected area by controlling the direction of the beam and increasing its path length.

Although intrusion Idetectors of this type have been highly successful, they are also subject to certain difliculties which the prior art has failed to solve. One of these problems arises from the placement of the units in high traic areas, as discussed above. The units are relatively large in size and and may be easily knocked out of alignment by persons accidentally striking the units as they walk past them. This is such a problem that at least one company has succesfully marketed fabricated frames which are rigidly installed around the units to guard them. A second problem arises from the existence of noise which may emanate from various sources, such as ambient illumination. 'In order to minimize the noise problem, the prior art units are designed to be highly directional so that the receiver will look at a minimum of ambient light. Because of this feature, ordinary vibrations in buildings can cause misalignment and false alarms. This feature also causes the units to be more susceptible to misalignment caused by their being accidentally struck by passersby, as explained above. The third difficulty, which is also allied with the highly directional characteristic just outlined, arises when mirrors are employed in the photoelectric system. It is well known to those skilled in the optical art that the beam retfected from a mirror will rotate through an angle equal to twice the angular rotation of the mirror. Thus, any vibration or misalignment to which the mirror is subjected will be amplified in the reifected beam. In practice, this problem is so severe that many installers will not utilize mirrors at all, in spite of their many advantages. In addition, a certain amount of beam attenuation will occur in the mirror due to absorption of the infrared energy.

The highly directional feature of prior art units also creates an economic problem in that installation must be 3,390,388 Patented June 25, 1968 effected by highly skilled personnel. This increases the installation costs and may reduce the economic advantages to be obtained from the intruder protection.

Accordingly, it is the primary object of the present invention to provide an improved photoelectric intrusion detector system. Other objects, features 4and advantages will be more apparent from the following description, the appended claims and the figures of the attached drawings, wherein:

FIG. l is a front view of a receiver usable in this invention, portions thereof being broken away to illustrate its internal construction;

FIG. 2 is a cross section taken along the line 2-2 of FIG. 1;

FIG. 3 is a front view of a transmitter unit usable in the present invention, portions thereof being broken away to illustrate its internal construction;

FIG. 4 is a cross section taken along the line 4--4 of FIG. 3;

FIG. 5 is a detail taken along the line 5-5 of FIG. 4;

FIG. 6 is an enlarged detail view taken along the line 66 of FIG. 4;

FIG. 7 is an isometric view of the filter assembly of this invention;

FIG. 8 is a cross section taken along the line 8-8 of FIG. 7;

FIG. 9 is an electrical schematic of the receiver and alarm portions ofthe system;

FIG. 10 are curves illustrating various features of the invention;

FIG. 11 is a front View of a mirror usable in this invention;

FIG. 12 is a cross section taken along the line 12-12 of FIG. 11, and

FIG. 13 is an optical schematic of a system in accordance with the invention employing plural mirrors.

The objects of this invention are achieved by reducing the size of the individual units while at the same time retaining the desirable aspects of prior art units and actually improving over their performance.

The one parameter which most strongly influences the size of the system units is the focal length of the optical system. In units constructed in accordance with the prior art, the focal length has ybeen reduced to approximiately six inches and two receiver configurations are in general use. These are the in-line system and the folded system. As its name implies, the in-line system employs a focusing lens and a photoampliier positioned along a single geometric axis which is also the optical axis of the system. Obviously, a receiver of this type must be at least six inches in length and, in practice, the overall length is closer to eleven inches to provide room for the photoamplier and the other parts. It will also be obvious that such a receiver cannot be recessed into a standard wall having 2 x 4" studs.

The folded system employs an angled mirror just behind the lens to fold the optical path downward, the photoamplifer being positioned generally below the lens. A receiver of this type does have a reduced front-to-back dimension, but its volume is increased in order to accommodate the mirror. A more serious objection is that a receiver of the folded type is highly responsive to any noise source located along a line projected from the photoamplier through the center of the lens. In order to minimize this difficulty, the receiver must be provided with a shade which projects from theI top of the unit and thereby partially defeats the size reduction gained by folding the optical path.

By means of the present invention there is provided a system having an in-line configuration of sharply reduced focal length. This has been achieved in spite of the many difficulties that would normally interfere with focal length reduction in photoelectric systems for operation at ranges of from D-250 feet.

T ransml'ttcr The transmitter T of this invention is illustrated in FIGS. 3-6. The transmitter comprises a U-shaped metallic chassis 10 including a front wall 10a, a bottom wall 10b and a back Wall 10c. The front wall 10a of the chassis defines a circular opening 12 and the back wall 10c defines a smaller but aligned circular opening 14. The bottom wall 10b denes a pair of openings, one including a plastic bushing 16 to permit entry of a power cable (not shown) and the other including an internally extending boss 1S which is internally threaded to receive a mounting bolt 20. The chassis 10 is mounted with its back wall 19C against a backing plate 22 which is substantially rectangular and includes a forwardly projecting rim 22a. The backing plate 22 is cut and formed to provide a pair of vertically spaced louvers 24 which are positioned over the opening 14 in chassis back wall 10c. As will be most apparent from FIG. 5, the opening 14 is enlarged :and its upper and lower edges form additional arcuate slots 14a.

Mounted against the back wall 10c of the' chassis and seated within the recess formed by opening 14 is a parabolic mirror 26. Mounted against the chassis 10c and below the mirror 26 is a lamp socket 28 in which is positioned a lamp 30 so that its filament is substantially vertical and positioned at the focal point of the mirror 26. Also mounted against the chassis back wall 10c is a power transformer 32 for energizing the lamp 30 from an external power source. As the wiring does not constitute part of this invention it is not illustrated in FIGS. 3 and 4, in order to simplify the showing.

The transmitter T is closed by means of a housing 34. The housing includes a front wall 34a, a bottom wall 3417 and three side walls 34e. The bottom wall 3417 defines a slot 36 which communicates with the back edge and is wider than the opening defined by bushing 16. The front wall defines a circular opening 38 and mounted on its inner surface and around the opening 38 is a mounting ring 40` securing an infrared filter 42 and a glass window 44. The housing 34 is positioned against the backing plate 22, as shown, with its back edge within the rim 22a. It is secured in this position by means of a spacer 46 and screw 48 which extends through the front wall 44a of the housing into a tapped opening in the front wall 10a of the chassis.

It will be noted that the transmitter disclosed in FIGS. 3-5 is much more compact than those known to the prior art. In actual practice, the lament of the lamp 30 is positioned approximately 1% inch from the surface of the mirror 26 and the entire unit, including the housing, is small enough that it can be recessed within a wall and not protrude beyond the standard 2 X 4 stud. The lamp 30 which is employed in the transmitter is of essentially the same rating as lamps employed in the much larger confines of prior art transmitters. A lamp such as this consumes approximately 2O watts but has certain advantages. These arise from the fact that the tungsten iilament is Sufiiciently large that its thermal inertia tends to cancel out any momentary fiuctuations or interruptions in its power supply. Not only must adequate cooling be provided, but light-tight integrity must be maintained, and this within an enclosure approximately of the volume of prior art enclosures.

To permit the entry of cooling air into the transmitter T the bottom wall 34]; has a slot 36 which provides an air passage between the bottom wall 34h and the chassfs bottom 10b. The transformer 32 is then positioned between this slot and the lamp 30. This serves as an effective barrier against the transmission of light through the air inlet. In order to provide a heated air outlet, novel use is made of the mirror 26 by allowing it to serve as n light hufllc as Well as a light reflector. This is achieved by providing the semi-circular openings 14a in the chassis back wall 10c which communicate with the openings of louvers 24. In this manner, a cooling air flow throughout the transmitter is achieved, as shown by the arrows in FIGS. 3 and 4. In addition, the inner surface of the housing 34 is coated with a reflective paint while its outer surface is provided with a dark wrinkle surface coating, as shown in FIG. 6. The wrinkle surface provides increased cooling surface area. The filter holder 40 is reflective and has a relatively large Contact surface with the housing 34 in order to conduct heat from the filter 42. In addition, the chassis is black anodized in order to absorb heat and is arranged in such a manner as to intercept a large percentage of the energy radiated in the vicinity of the infrared filter. The chassis, backing plate, and cover are of high thermally conductive materials, such as aluminum.

The transmitter may be secured to a wall or other supporting surface by means of a mounting bracket 50. Bracket 50 is substantially L-shaped and includes a vertical bifurcated back 52 which is secured to the wall by means of suitable screws 54, and a horizontal support shelf 56 which includes a centrally positioned boss 58 which extends through the slot 35 in housing 35 and into which is threaded the mounting bolt 20. The support shelf also defines openings 60 aligned with the openings in the bushing 16. It is important to note that, because lof the vast reduction in size of the transmitter T, the bracket can actually be constructed with its horizontal support shelf 56 extending beyond the limits of the transmitter. This provides considerable additional protection against accidental striking of the unit.

Receiver The receiver R employed with this invention is illustrated in FIGS. 1 and 2. In external size and appearance it is quite similar to the transmitter described above. It comprises an L-shaped chassis 69 including a front wall 60a, a bottom Wall 6012 and a rear flange 60C. Like the transmitter chassis, the receiver chassis includes a raised boss -62 by which the chassis is mounted to a similar bracket 50 by means of a mounting bolt 20. The .ti'ange 60C is secured to a backing plate 64 having an outer rim 64a. A mounting board 66 is attached to the backing plate by means of bolts 68, nuts and spacers 72.

Mounted against the mounting board 66 are a filter assembly F, a photocell 116, a terminal board B, and the various electronic components to be described infra. The front wall 60a of chassis 60 defines a circular window opening 74 and a rectangular meter -opening 76. A meter 78 is mounted against the front wall 60a so as to protrude from the meter opening, as illustrated. A short focal length lens Si) is mounted by means of a holder 82 in front of the window opening 74.

A housing 84 including a front wall 84a, a bottom Wall 8417 and side walls 84C is positioned within the rim 64a in the same maner as for the transmitter. The housing is secured to the chassis by means of bolt 86 and spacer 88.

In addition to the receiver, one of the novel aspects of the mounting bracket 50 is also illustrated in FIG. 2. The brackets are designed in such a manner that the backs 52 are shortened suiiiciently that a second mounting ybracket 50 may be reversed and mounted above the receiver or transmitter unit to provide additional mechanical protection.

The filter assembly F is illustrated in detail in FIGS. 7 and 8. As previously described, the focal length of the transmitter is approximately /i inch. The combination of short focal length and large filament projects an extremely large beam at the receiver location. As described above, the lamp 30 is positioned in such a manner that its filament is substantially vertical, Thus, the beam has a greater vertical than horizontal dimension. For this reason, no vertical alignment of the transmitter is required upon installation, even though, as is a common practice,

the receiver is mounted at a higher level than the transmitter. The receiver need only be mounted within the beam. This eliminates the need for installation by skilled personnel. The filter assembly is so constructed that the optical axis between the transmitter and receiver may be tilted for maximum intensity of the final image in a very simple manner. To this end, the filter assembly comprises a backing plate 90 in the form of a hollow rectangular frame surrounding photocell 116 and defining a slot 92 at one end of its top surface. Mounted upon the backing plate 90 is a spacer 94 which is also in the form of a hollow rectangular frame, the bottom of the spacer being aligned with the bottom of the backing plate and its upper surface level with the bottom of slot 92. Mounted against the front surface of the spacer 94 is a filter plate 96. The filter plate is of essentially the same outer dimensions as the backing plate 90 and defines a slot 98 which is aligned with slot 92. The filter plate 96 further defines a cent-rally positioned circular field stop 100 which serves as a spatial filter. A pivot screw 102 (FIG. 8) extends through openings in plates 90, 94 and through a larger pivot opening 95 in filter plate 96. A cylindrical bushing 97 is positioned in pivot opening 95 and against plate 94. A split lock Washer 93 is also mounted against filter plate 96 around bushing 97 and extends beyond the bushing. The assembly is secured by a washer 91, a lock Washer 99', and a nut 101. Tightening of nut 101 compresses split washer 93 and provides the desired compressive force across the assembly. The right end of the filter plate defines a curved limit slot 106 concentric with the pivot screw 102. A screw 108 extends through the limit slot 106. A nut 107 and a bent washer 110 provide a resilient bearing force against the front of the filter plate 96. As explained, no vertical alignment should be required under normal conditions. However, in the event of poor 4installation which makes it difiicult to align the minimum circle of confusion with the field stop 100, it is simply necessary to insert a screw driver blade 112 into the slots 92, 98. A twist of the screw driver will then pivot the filter plate 96 about the -pivot screw 102, permitting vertical alignment of the field stop 100. Horizontal alignment is provided by the mounting bolts which secure the transmitter and receiver to their respective mounting brackets. An infrared filter 114 is mounted on the back surface of the filter plate 96 behind the field 1stop 100. When using a B1 inch focal length transmitter mirror 26 and a 2" focal Ilength receiver lens 80, the field stop 100 may be approximately 5%4 in diameter. The image of this field stop, projected on the transmitter, has sufficient vertical dimension at all practical ranges to insure signal reception without vertical alignment.

Filter selection In order t-o eliminate the problem of receiver noise, spectral, as well as spatial, filtering is desirable. The two main sources of receiver noise arise from artificial ambient light and refiected ambient sunlight. The filtering to be described is based on the use of a cadmium selenide photoconductive cel'l. The spectral response of such a cell 116 is illustrated in FIG. l0 as curve 118, Fluorescent lighting is the most popular and efiicient artificial lighting being used today. The spectral characteristics of this lighting are illustrated by curve 120. The spectral ch-aracteristics of sunlight are illustrated by curve 122. In order to eliminate the maximum possible noise, a filter 114 having the spectral transmission characteristics shown by curve 124 was employed. This filter almost completely rejects the main source of noise-fluorescent lights. It also provides the equivalent of an imped-ance match between the transmitter and the receiver by exactly matching their spectral responses and rejects most of the solar energy. The filter is also economically advantageous because it need 'be only slightly larger than the field stop 100, since it is installed at the receiver focal plane instead of at its aperture. It will be noted that the spectral characteristic of the filter is not matched t-o that of the cell, resulting in some loss of sensitivity. However, the cell response peaks very sharply so that the area under the peak is not large. Thus the loss is not great and the exclusion of the fluorescent light spectrum is highly advantageous. Noise was further eliminated by the use of a wrinkle paint finish on the inside of the receiver. This assists in reducing noise by increasing the scattering of the retiective light and by selectively absorbing the infrared lportion of any unwanted radiation.

Electronic circuitry The circuitry of the receiver is illustrated in FIG. 9. The receiver R is illustrated as receiving light from the transmitter T and a mirror M ywhich is focused through the lens onto the cell 116. The receiver is powered by a standard 1.5 volt dry cell 126. The emitter of a silicon transistor 128 is connected to one side of the dry cell 126 and the collector is connected through a relay coil 130 and the meter 75 to the other side of the dry cell.

Underwriters Laboratory specifications require that units of the type disclosed herein should operate over a temperature range of 30-120 F. The amplifier should ideally have a cutoff which is independent of the temperature, because the cutoff point determines the range 0f the system. Accordingly, one problem which has faced the manufacturer of a receiver of this type is -to achieve bias stability at minimum cost and power consumption. Two meth-ods are presently employed in prior art systems to achieve this objective. One method utilizes a resistor in the emitter which introduces negative feedback to c-ounteract the rise in collector current with temperature. This has two disadvantages-namely, a loss in gain and a loss in the voltage available to energize the relay. At saturation, the full battery supply voltage should be kept available to operate the relay coil and overcome the internal resistance of the meter. A second approach is to utilize an element which has a high negative temperature coefficient, such as a thermistor, in the biasing network instead of a fixed resistor. The disadvantage of this method is the cost of the thermistor, which can equal or exceed the cost of the transistor itself. In accordance with the present invention, the high positive temperature coefiicient of the cell 116 itself was utilized by adding a simple carbon resistor 132 rated at 15,000 ohms in series with the cell to scale the overall temperature coefficient of the series combination to the level needed to compensate for the drift of the transistor with temperature. The resistor 134 is rated at approximately 47,000 ohms. As long as the cell 116 receives the proper infrared radiation from the lens 80, the relay coil 130 remains energized, keeping the relay contacts 136 in their open position. lOn interruption of the radiation, the relay coil is de-energized, allowing the contacts to close and lactuate the alarm 138 from the power source 140.

' Mirror As has been previously explained, it would often be desirable to use mirrors in an intrusion detector system. The unexpected advantages achieved by the novel transmitter and receiver construction disclosed above permit the use of a mirror in the present system, while still retaining reduced sensitivity to misalignment in comparison with a prior art system having no mirrors.

In FIGS. ll and l2 there is illustrated a mirror construction Which utilizes the same mounting bracket 50 and bolt 20 as is employed for the transmitter and receiver. The mirror assembly requires no vertical alignment of the brackets, but incorporates a novel spring suspension system which permits vertical adjustment by a self-locking adjusting screw without loosening or tightening any additional screws, nuts or bolts.

A rectangular housing 142, having an open front and a mounting boss 144, is mounted on the bracket 50. A substantially rectangular mounting plate 146 is provided with a pair of horizontally aligned lower mounting holes through which extend a pair of mounting screws 148 which are threaded into the back of the housing 142. The -rnounting plate 146 is spaced from the housing by means of a spacer 150 on each of the mounting screws. A coil spring 152 is arranged around each spacer to provide outward force against the mounting plate 146. The shank of each of the screws 148 is smaller than its corresponding opening in mounting plate 146 to permit tilting of the mounting plate. A vertical adjustment screw 154 extends through a centrally positioned opening in the top of the mounting plate 146 and into the back of the housing 142. A spring 156 exerts outward pressure `against the top part of the mounting plate 146. A mirror 158 is adhesively secured to the front of the mounting plate 146. The adjusting screw 154 is of the self-locking type having a plastic insert extending into its threads to prevent unintentional rotation. It will be seen that vertical adjustment of the mirror 158 about the axis defined by screws 148 may be achieved by simply rotating the adjusting screw 154. An inverted lbracket f) may also be used to protect the top of the mirror.

An overall system in accordance with this invention employing two mirrors is illustrated in FIG. 13. The filament of lamp 30 is positioned a distance a from the parabolic mirror 26 equal to the focal length of the mirror. In the described embodiment this is The illumination from the lamp filament is collimated and passes through the filter 4Z and across a region to be protected to 'a first mirror M1. Mirror M1 directs the illumination to a second mirror M2 which then directs the collimated radiation to the lens 80 of the receiver. Lens 80 has a focal length b, in the illustrated example 2 inches, at which is positioned a filter plate 96 having a field stop therein backed by a second infrared filter 114. The photocell 116 receives illumination from lens 80 which passes through both the field stop and the infrared filter.

By means of the present invention, all of the objectives hereinbefore set forth have been achieved. It will be apparent to those skilled in the art that various changes and modifications may be made in this invention without departing from its spirit and scope. Accordingly, it is to be understood that the foregoing description is illustrative only, rather than limiting. This invention is limited only by the scope of the following claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1, A transmitter for an infrared photoelectric detection system which comprises: an enclosure including a bottom wall defining an air inlet therein, a back wall defining an air outlet in the upper portion thereof, and a front wall defining a radiation window therethrough; a focusing mirror positioned with its convex surface adjacent said back wall in light blocking relationship to said air outlet; infrared radiation generating means positioned at the focal point of said mirror to project radiation from said generating means and mirror through said window; transformer means supplying said radiation generating means and positioned in light blocking relationship to said air inlet; and infrared filter means positioned in said window.

2. The transmitter of claim 1 wherein said radiation generating means is an electric lamp.

3. The transmitter of claim 2 wherein the long dimension of the filament of said lamp is vertically oriented.

4. A transmitter for an infrared photoelectric detection system which comprises: mounting means; an enclosure positioned on said mounting means for selective rotation about a vertical axis, said enclosure including a bottom wall defining an air inlet therein, a back wall defining an air outlet in the upper portion thereof, and a front wall defining a radiation window therethrough; a concave focusing mirror positioned with its convex surface adjacent said back wall in light blocking relationship to said air outlet; electric lamp means positioned at the focal point of said mirror to project light from said lamp means and mirror through said window; transformer means supplying said lamp means and positioned between said lamp means and air inlet in light blocking relationship to said air inlet; and infrared filter means positioned in said window.

5. The transmitter of claim 4 wherein said filter means is essentially opaque to radiation having wavelengths shorter than approximately 8000 Angstrom units.

6. The transmitter of claim 4 wherein said mounting means is an essentially L-shaped bracket comprising a horizontally extending shelf portion of a size sufficient to extend beyond said enclosure; a vertical mounting screw interconnecting said shelf portion and said enclosure; and a wall mounting portion having a vertical dimension approximately half the height of said enclosure.

7. A receiver for an infrared photoelectric detection system which comprises: an enclosure including a front wall defining a radiation window therethrough; a focusing lens in said window arranged to focus collimated radiation at a focal plane within said enclosure; a filter assembly substantially at said focal plane, said filter assembly. including a base member, and a filter plate member, said filter plate member defining a field stop therein; an infrared filter covering said field stop; photoelectric detector means positioned to receive radiation passing through said field stop; and indicating means responsive to Said photoelectric detector means to indicate an interruption of Said radiation.

8. The receiver of claim 7 wherein said base member comprises: a substantially rectangular frame defining a central opening therethrough and a first adjusting slot in one edge; and a spacing member mounted against said frame and having one edge essentially coplanar with the innermost surface of said adjusting slot; and wherein Said filter plate member is pivotally mounted against said spacing member and further defines a second adjusting slot in one edge aligned with said first adjusting slot.

9. The receiver of claim 7 wherein said indicating means comprises: a first resistor having a positive temperature coefficient of resistance connected in a Series combination with said detector means; a second, biasing, resistor in series with said series combination; D.C. power supply means connected across said series combination and said second resistor; transistor means having its base connected between said series combination and said second resistor, the collector-emitter circuit being connected across said D.C. power supply; a relay coil in series with said collector-emitter circuit; relay contacts operable by said relay coil; and alarm means in series with said relay contacts.

10. The receiver of claim 9 wherein said photoelectric detector means is a cadmium selenide photocell.

11. An infrared photoelectric detection system which comprises: a transmitter enclosure positioned for selective rotation about a vertical axis and having a bottom wall defining an air inlet therein, a back Wall defining an air outlet in the upper portion thereof, land a front wall defining a radiation transmitting window therethrough; a concave collimating mirror within said transmitter enclosure positioned with its convex surface adjacent said back wall in light blocking relationship to said air outlet; electric lamp means within said transmitter enclosure at the focal point of said mirror to project collimated light from said lamp means and mirror through said window; transformer means in said transmitter enclosure supplying said lamp means and positioned between said lamp means and air inlet in light blocking relationship to said air inlet; first infrared filter means in said transmitting window essentially opaque to radiation having wavelengths shorter than approximately 8000 Angstrom units; a receiver enclosure spaced from said transmitter enclosure and positioned for selective rotation about a vertical axis, said receiver enclosure including a front wall defining a radiation receiving window therethrough; a focusing lens in said radiation receiving window arranged to focus collimated light at a focal plane within said receiving enclosure; a substantially rectangular frame within said receiver enclosure defining a central opening therethrough and a first adjusting slot in one edge; a spacing member mounted against said frame and having one edge essentially cplanar with the innermost surface of said adjusting slot; a filter plate member pivotally mounted against said spacing member at said focal plane defining a field stop therein and a second adjusting slot in one edge aligned with said first adjusting slot; second infrared filter means over said field stop essentially opaque to radiation wavelengths shorter than approximately 8000 Angstrom units; photoelectric detector means in said receiver enclosure positioned to receive radiation passing through said field stop and said second filter means; and indicating means responsive to said photoelectric detector means to indicate an interruption of said radiation.

12. The system of claim 11 wherein mirror means is positioned to direct said collimated light from said transmitter enclosure to said receiver enclosure.

13. The system of claim 12 wherein said mirror means comprises: a mounting bracket; a housing on said mounting bracket rotatable about a vertical axis; a mirror; mirror support means intermediate said mirror and housing retaining said mirror against said housing for rotation about a horizontal axis relative thereto; and adjusting screw means intermediate said mirror and housing and spaced from said horizontal axis to pivot said mirror about said horizontal axis.

14. A mirror for a photoelectric detection system which comprises: a mounting bracket; a housing on said mounting bracket rotatable about a vertical axis; a mirror; mirror support means intermediate said mirror and housing retain-ing said mirror against said housing for rotation about a horizontal axis relative thereto; and ad- 10 justing screw means intermediate said mirror and housing and spaced from said horizontal axis to pivot said mirror about said horizontal axis.

15. The mirror of claim 14 wherein said mirror support means comprises: a substantially rectangular mounting plate defining first and second spaced pivot openings along its lower edge and an adjusting opening along its upper edge; rst and second pivot screws passing, respectively, through said pivot openings and into said housing, the diameter of said pivot screws being smaller than the diameter of said pivot openings; and spacing means between said lower edge and said housing.

16. The mirror of claim 15 wherein said adjusting screw means extends between said adjusting open-ing and ,said housing and includes spring means resiliently retaining said upper edge from said housing.

References Cited UNITED STATES PATENTS 1,345,793 7/ 1920 Marks 240-47 1,694,833 12/1928 Shively 248-487 2,099,868 11/1937 Sing et al.

2,147,156 2/1939 Geffcken et al. 250-239 X 2,207,097 7/ 1940 Logan 250-239 2,278,936 4/1942 Lindsay et al. 340-228 X 2,487,024 11/ 1949 Mathison 340-228 X 2,918,585 12/1959 Farmer Z50-239 3,293,504 12/-1966 Percival 250-239 X FOREIGN PATENTS 671,973 5/1952 Great Britain.

NEIL C. READ, Primary Examiner.

D. L. TRAFTON, Assistant Examiner. 

1. A TRANSMITTER FOR AN INFRARED PHOTOELECTRIC DETECTION SYSTEM WHICH COMPRISES: AN ENCLOSURE INCLUDING A BOTTOM WALL DEFINING AN AIR INLET THEREIN, A BACK WALL DEFINING AN AIR OUTLET IN THE UPPER PORTION THEREOF, AND A FRONT WALL DEFINING A RADIATION WINDOW THERETHROUGH; A FOCUSING MIRROR POSITIONED WITH ITS CONVEX SURFACE ADJACENT SAID BACK WALL IN LIGHT BLOCKING RELATIONSHIP TO SAID AIR OUTLET; INFRARED RADIATION GENERATING MEANS POSI- 